1. Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image formed on an image bearing member or a cartridge having the developing device, detachably mountable to an image forming apparatus. In particular, the present invention relates to a developing device or a cartridge used in an image forming apparatus such as a copier, a printer, and a facsimile.
2. Description of the Related Art
Conventionally, as a developing device used in an image forming apparatus using an electrophotographic process, various devices have been proposed and put into practical use. Such devices are roughly classified into a developing device of a mono-component development system and a developing device of a dual-component development system. The mono-component development system mostly adopts a non-contact system, and as a representative development method, there is a mono-component jumping development method using magnetic toner.
According to this development method, a high quality image can be obtained with a simple configuration. However, toner contains a magnetic substance, so that a color image cannot be obtained. According to the mono-component development method using non-magnetic toner, a color image can be obtained. However, it is difficult to coat a developing sleeve with toner. Therefore, the developing sleeve is coated with toner with an elastic blade, which is disadvantageous in terms of stability and durability.
According to the dual-component development method, toner is fed to a development region by magnetic carriers for development. A development process is usually conducted by bringing a developer into contact with a photosensitive drum. Hereinafter, the development process will be described with reference to FIG. 6.
In FIG. 6, reference numeral 30 denotes a developing sleeve, 35 denotes a magnet roller fixed in the developing sleeve, 31 and 32 denote agitating screws, 33 denotes a regulating blade disposed so as to form a developer into a thin layer on the surface of the developing sleeve, and 34 denotes a developing container. Now, a development process of visualizing an electrostatic latent image with a developing device by a dual-component magnetic brush method, and a circulation system of a developer will be described. A developer taken up at an N3-pole along with the rotation of the developing sleeve 30 is regulated by the regulating blade 33 while being fed from an S2-pole to an N1-pole, and formed into a thin layer on the developing sleeve 30.
When the developer formed into a thin layer is fed to an S1-pole that is a development main pole, a magnetic brush is formed due to a magnetic force. An electrostatic latent image is developed with the developer which stands like ears of rice. Thereafter, the developer on the developing sleeve 30 is returned to the developing container 34 by a repulsive magnetic field of an N2-pole and an N3-pole. The dual-component development is generally conducted in such a manner that, as described above, magnetic poles with the same polarity are arranged, and a developer after development is peeled from a developing sleeve so as not to allow an image history to remain.
The developing sleeve is supplied with a D.C. bias and an A.C. bias from a power supply (not shown). In general, according to the dual-component development method, application of an A.C. bias enhances a development efficiency, resulting in a high quality image.
As a method for forming an electrostatic latent image, there is a known method for scanning an electrophotographic photosensitive member with a laser beam modulated in accordance with an image signal to be recorded, thereby exposing the member to light, and forming an electrostatic latent image in which a latent image having a dot distribution shape (i.e., a dot-shaped latent image) is distributed in accordance with an image. In particular, according to a so-called pulse width modulation (PWM) method for modulating a width (i.e., a duration time) of a driving pulse current of a laser in accordance with a tone of an image to be recorded, a high recording density (i.e., a high resolution) and high gradations can be obtained.
In recent years, higher quality and longer life using a dual-component developing device have been developed. In order to achieve a long life, it is required to prevent a developer from being compressed, so as to avoid degradation of toner and carriers. As one method, decreasing magnetization of magnetic carriers in a developer is considered. Decreasing magnetization of carriers weakens a force of rubbing a toner image developed onto a photoconductor in a developing section, and thus results in high quality image.
In the case where the magnetization of carriers is decreased, and the developing sleeve and the photosensitive drum rotate in a forward direction, there is also a disadvantage while the above-mentioned advantage is obtained. More specifically, in a developing section, a magnetic brush becomes short, and a nip (peripheral direction) at which a developer is in contact with the photosensitive drum becomes narrow. As a result, the density of a trailing end of a black copy is increased; that is, edge enhancement such as so-called sweeping-together becomes conspicuous.
This phenomenon occurs as follows. In the case where the developing sleeve and the photosensitive drum rotate in a forward direction at an opposing portion, during development of a black copy, toner accumulates on the photosensitive drum side, upstream of the above-mentioned nip at which a developer is in contact with the photosensitive drum, and toner hardly adheres to the photosensitive drum in a solid black area due to development defects. Consequently, accumulated toner adheres to the trailing end (of the black copy) at once.
When a magnetic brush is long, the nip at which the magnetic brush is in contact with the photosensitive drum becomes longer. Therefore, toner does not accumulate, and sweeping-together at the trailing end of a black copy does not occur. Furthermore, in counter development in which the developing sleeve and the photosensitive drum rotate in opposite directions at an opposing portion, toner does not accumulate on the photosensitive drum side, upstream of the above-mentioned nip. Therefore, even when the magnetization of carriers is decreased, and a magnetic brush is shortened, edge enhancement such as sweeping-together is unlikely to occur.
Accumulation phenomenon in the case where the developing sleeve and the photosensitive drum rotate in a forward direction occurs on the photosensitive drum. More specifically, this phenomenon occurs upstream in a rotation direction of the photosensitive drum (upstream in a rotation direction of the developing sleeve) from the nip at which a magnetic brush is in contact with the photosensitive drum. However, in counter development, the developing sleeve and the photosensitive drum move in opposite directions. Therefore, toner actually does not accumulate and is fed outside of the nip by the rotation of the photosensitive drum, whereby the accumulation of toner that causes sweeping-together and the like does not occur.
In producing and assembling a developing device, conductive magnetic powder may enter a developing container. For example, in the case where a developing container with a developing sleeve attached thereto is closed with a cover to complete a developing device, when the cover is fixed onto the developing container with a screw, chip powder of the screw (i.e., conductive magnetic powder) formed between the screw and a bit insert may enter the developing container. Conductive magnetic power may adhere to the clothing of an operator who assembles the developing container or an operation tool (e.g., a screwdriver, etc.), and this powder may enter the developing container during assembling. Thus, in a production and assembly process of the developing device (developing container), and in an exchange process of the developing device (developing container), conductive magnetic powder may enter the developing device.
In the case where a primary charger adopting a corona charging system is used for charging a photoconductor so as to form an electrostatic latent image, conductive magnetic powder may also enter the developing container. More specifically, in order to refresh a discharge wire of the primary charger (i.e., to enhance a discharge efficiency by removing a substance generated by discharge, which may adhere to the discharge wire due to repeated use), the discharge wire is ground manually by a user or a serviceman or automatically to generate conductive magnetic power. This conductive magnetic power may enter the developing container for some reason.
However, in the case where development is conducted in the above-mentioned constitution in which the magnetization of carriers is decreased, and the developing sleeve and the photosensitive drum are allowed to rotate in opposite directions at an opposing portion, when conductive magnetic powder enters the developing container, discharge marks are generated continuously in a line shape. The maximum length of the discharge mark thus generated in a line shape sometimes reach 700 mm.
In the discharge phenomenon, discharge marks are hardly generated continuously, but formed individually. Individual discharge marks are formed in such a manner that a white hollow portion is formed at the center and the periphery thereof is thick in a ring shape, or the discharge marks become thick entirely. The discharge marks are generated in both a blank copy area and a black copy area. When the discharge marks are generated continuously in a line shape, they become conspicuous, which substantially degrades image quality.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a developing device in which an occurrence ratio of discharge marks can be decreased even when conductive magnetic powder enters a developing container.
Another object of the present invention is to provide a developing device in which the maximum length of discharge marks can be reduced even when they are generated in a line shape.
Still another object of the present invention is to provide a cartridge in which an occurrence ratio of discharge marks can be decreased even when conductive magnetic powder enters a developing container.
Still another object of the present invention is to provide a cartridge in which the maximum length of discharge marks can be reduced even when they are generated in a line shape.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.